An optical receiver module to receive a wavelength multiplexed light often installs a plurality of photodiodes (PDs) within a unique housing with an optical de-multiplexer to de-multiplex the wavelength multiplexed light into a plurality of optical signals each having a specific wavelength different from others. One type of the optical de-multiplexer has a plurality of wavelength selective filters (WSFs) and a plurality of reflectors sequentially disposed along the optical path thereof. The WSFs each transmits an optical signal with a wavelength specific thereto and reflects other optical signals.
For such an optical de-multiplexer, an incident angle of the wavelength multiplexed light becomes a key factor to output de-multiplexed optical signals uniformly, because optical paths from the input port of the optical de-multiplexer to respective WSFs are not uniform, and a WSF is influenced by the incident angle of the wavelength multiplexed light as it is apart from the input port on the optical path. One reason to cause a deviation in the incident angle of the wavelength multiplexed light into the optical de-multiplexer is how to fix the coupling unit to the front wall of the housing of the module. The front wall of the housing often provides a bush with an opening, through which the wavelength multiplexed light passes, to make the fixation of the coupling unit by the laser welding. Such a two-body structure of the bush and the housing becomes hard to set the angle of the front wall, namely, the surface of the bush, within an acceptable range of ±0.5° around the designed angle.
As the transmission speed of the optical signal increases, a pre-amplifier that converts a photocurrent generated by a PD into a voltage signal becomes necessary to be mounted immediate to the PD. Moreover, when an optical module like the present application receives a wavelength multiplexed signal, a plurality of pre-amplifiers is necessary to be installed immediate to the PDs within the housing. These two reasons of the increase of the transmission speed and the installation of the plural pre-amplifiers cause greater power consumption by the pre-amplifiers, which means that the pre-amplifiers are preferably placed immediate to the PDs on the bottom of the housing to enhance the efficiency of the heat dissipation; accordingly, the PDs are also mounted on the bottom so as to face the sensing surface thereof upward. A specific arrangement of the optical de-multiplexer is necessary to guide the optical signals de-multiplexed thereby to the PDs on the bottom of the housing.